pubs.acs.org/MacromoleculesPublished on Web 07/17/2009r 2009 American Chemical Society

Macromolecules 2009, 42, 6107–6114 6107

DOI: 10.1021/ma900968f

Design and Synthesis of Alternating Regioregular Oligothiophenes/Benzothiadiazole Copolymers for Organic Solar Cells

Fushun Liang,†,‡,§ Jianping Lu,*,‡ Jianfu Ding,*,† Raluca Movileanu,‡ and Ye Tao‡

†Institute for Chemical Process and Environmental Technology (ICPET) and ‡Institute for MicrostructuralSciences (IMS), National Research Council of Canada (NRC), 1200 Montreal Road, Ottawa, ON K1A 0R6,Canada. §Current address: Department of Chemistry, Northeast Normal University, 5268 Renmin St.,Changchun 130024, China.

Received May 4, 2009; Revised Manuscript Received July 6, 2009

ABSTRACT: Low-bandgap regioregular polythiophene derivatives, PTh6BTD, PTh8BTD, andPTh4TTBTD, were synthesized through Stille coupling reaction. These are alternating copolymers of anelectron-deficient benzothiadiazole unit and oligothiophene units including hexathiophene, octathiophene,and thieno[3,2-b]thiophene-bridged quarterthiophene, respectively. The polymers are soluble in halogenatedsolvents such as o-dichlorobenzene, affording good processability in solar cell fabrication. Meanwhile, thesynthesized alternating copolymers show much broader absorption than P3HT, covering the spectral regionfrom 350 to 800 nm. DSC analysis showed that all three polymers readily crystallized, indicating highlyordered intermolecular packing, which is beneficial for achieving higher charge carrier mobility. Bulk-heterojunction solar cells using 1:1 w/w PThnBTD:PC61BM ([6,6]-phenyl C61-butyric acid methyl ester)blends as the photovoltaic active layers were fabricated and characterized. Best power conversion efficiencyof 1.7% was obtained from PTh8BTD-based devices under a simulated AM 1.5 G solar irradiation of100 mW/cm2.

Introduction

Despite facing challenges in long-term stability,1 organic solarcells based on conjugated polymers and soluble fullerene deriva-tives have attracted a great deal of attention due to their solutionprocessability, mechanical flexibility, and the potential for low-cost large-area manufacturing.2 Recently, power conversionefficiencies in the 5% range have been reported independentlyby several groups based on poly(3-hexylthiophene)/[6,6]-phenylC61 butyric acid methyl ester (P3HT:PC61BM) bulk heterojunc-tion (BHJ) solar cells.3-6 Although P3HT is one of the mostpromising materials ever tested in polymeric solar cells, themismatch between its relatively large band gap (2.0 eV) and theenergy distribution of the solar spectrum leads to insufficient lightabsorption and limits the further improvement in the powerconversion efficiency. Hence, it is necessary to lower the bandgapto move the absorption toward longer wavelengths. In thisregard, low-bandgap polymers based on internal electron do-nor-acceptor (D-A) interaction have been developed. Forinstance, in 2006, Inganaes et al. reported low-bandgap alternat-ing fluorene copolymers containing electron-withdrawing thie-noxaline;7 Muehlbacher presented a copolymer of cyclopenta-dithiophene and 2,1,3-benzothiadiazole with a low bandgap of1.73 eV;8 and more recently, Leclerc and co-workers developed alow-bandgap poly(2,7-carbazole) derivative demonstrating apower conversion efficiency of 3.6%when blended with PC61BMas the photovoltaic (PV) active layer, and the efficiency wasfurther improved to 6.1% after device optimization.9

On the other hand, due to strong interchain interaction inthe solid state, some representative polythiophene systems likepoly(3-hexylthiophene) (P3HT),3,5 polyterthiophene (PTT),10

polyquarterthiophene (PQT),11 and poly(2,5-bis(3-alkylthio-phen-2-yl)thieno[3,2-b]thiophene) (PBTT)12 show high chargecarrier mobility, which is a very attractive feature and criticallyimportant for high-performance solar cells. Our strategy todevelop low-bandgap polymers with high charge carrier mobilityis to introduce a frequently used electron-deficient 2,1,3-ben-zothiadiazole (BTD) unit into the polythiophene main chain tolower the bandgap and to increase the number of thiophene ringsin the oligothiophene segments for better interchain packing andsolubility. With these in mind, 2,1,3-benzothiadiazole-coredregioregular oligothiophene monomers were designed andsynthesized. Our recent work demonstrated that these oligothio-phene monomers can be used as novel building blocks toconstruct crystalline polymeric materials for solar cell applica-tions via copolymerization with other π-conjugated compoundssuch as indolo[3,2-b]carbazole.13 In this work, these oligothio-phenes were copolymerized with 5,50-bis(stannyl)-2,20-bithio-phene or 2,5-bis(stannyl)thieno[3,2-b]thiophene via Stillereaction, resulting in three crystalline alternative copolymers,PTh6BTD, PTh8BTD, and PTh4TTBTD, as shown in Scheme 1.The length of the oligothiophene unit varied from 6 to 8thiophene rings and thieno[3,2-b]thiophene-bridged 4 thiophenerings. Toour knowledge, there are a few examples in the literatureon the low-bandgap copolymers consisting of oligothiophenesand electron acceptors (quinoxaline,14 oxadiazole,15 benzothia-diazole,16 or thiazolothiazole17).However, themaximumnumberof thiophene rings in the oligothiophene segments is nomore than4. Some of these polymers exhibited poor solubility in commonorganic solvents due to the lack of enough amount of alkyl sidechains, and some polymers were synthesized via the oxidativepolymerization method, in which the oxidant needs to be re-moved. In the previous work on the copolymers of oligothio-phenes andbenzothiadiazole, the best efficiency achievedwas lessthan 1%.16

*Towhomall correspondence shouldbe addressed: Ph (613)990-1651;Fax (613)990-0202; e-mail Jianping.Lu@nrc-cnrc.gc.ca, Jianfu.Ding@nrc-cnrc.gc.ca.

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In this paper, we present the synthesis of PTh6BTD,PTh8BTD, and PTh4TTBTD, followed by a detailed discussionon their optical, thermal, electrochemical, and photovoltaicproperties, and demonstrate that with rational design of thesubstitution pattern of the substituents, low-bandgap semicon-ducting polymers with both high crystallinity and good solubilitycan beobtained.The initial study of the bulk-heterojunction solarcells using 1:1 w/w PTh8BTD:PC61BM blend as the PV activelayer showed a power conversion efficiency of 1.7% under asimulated AM 1.5 G solar irradiation at 100 mW/cm2.

Results and Discussion

Synthesis. Two key monomers 4 and 6, dibrominatedoligothiophenes incorporating benzothiadiazole in the center,were prepared in a divergentmethod from4,7-dibromobenzo-thiadiazole via a repetitive Stille coupling and NBS bromina-tion procedure (Scheme 2). We also attempted to synthe-size quarterthiophene end-capped benzothiadiazole from

compound 6 but failed due to the purification problem. It isworth pointing out that the bromination condition was dif-ferent for each cycle in order to get high bromination yield atthe desired position. For example, 4,7-dibromo-2,1,3-benzo-thiadiazole was synthesized using liquid Br2 as the bromina-tion agent in 47%HBr solutionwith refluxing,while 4,7-bis(4-octyl-2-thienyl)-2,1,3-benzothiadiazole was readily bromi-nated with N-bromosuccinimide (NBS) at room temperature

Scheme 1. Chemical Structures Reported in This Work

Figure 1. GPC curves of polymers PTh6BTD and PTh8BTD.PTh6BTD was first dissolved in chlorobenzene and then diluted withTHF.

Scheme 2. Synthetic Routes toward Copolymers PTh6BTD,PTh8BTD, and PTh4TTBTD

a

aReagents and conditions: (i) Pd(PPh3)4, toluene, Ar, 110 �C; (ii)NBS, CH2Cl2, rt; (iii) NBS, CH2Cl2, -10 �C.

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in the presence of silica gel as a catalyst. In contrast, thebromination of 4,7-bis(3,40,400-trioctyl-2,20:50,200-terthiophen-5-yl)-2,1,3-benzothiadiazole should be performed under evenmilder condition, such as at-10 �C. Otherwise, brominationwould also occur at other positions, leading to the formationof a mixture that is difficult to purify.

All three polymers PTh6BTD, PTh8BTD, andPTh4TTBTD were successfully synthesized by Stille cou-pling between 5,50-bis(trimethylstannyl)-2,20-bithiophene or

2,5-bis(stannyl)thieno[3,2-b]thiophene and appropriate di-bromo compounds in anhydrous toluene at 110 �C for 36 h.18

Since the palladium catalyst is quite sensitive to oxygen andlight, the polymerization reaction was run in the dark using astandard Schlenk technique. To increase the stability ofthe polymers, end-capping reactions were performed using

Figure 2. Absorption spectra of polymers PTh6BTD, PTh8BTD, andPTh4TTBTD in 1,2-dicholorobenzene solution and in solid film state(50 nm thick). The annealing temperatures for PTh6BTD, PTh8BTD,and PTh4TTBTD films are 120, 100, and 200 �C, respectively.

Figure 3. Second scan DSC curves of PTh6BTD and PTh8BTD.PTh4TTBTD at a heating/cooling rate of 10 �C under nitrogen.

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bromobenzene and 2-stannylthiophene. The polymers werepurified through Soxhlet extraction in refluxing acetone for12 h and then in hexane for an additional 12 h to removeoligomers. PTh4TTBTD was further extracted with THF.Finally, high-molecular-weight polymers were extractedwith chlorobenzene, concentrated, and precipitated inmethanol. Black fibers for three polymers (PTh6BTD,PTh8BTD, and PTh4TTBTD) were obtained in moderateyields of 49%, 63%, and 45%, respectively. PTh8BTD wassoluble in refluxing toluene, and no polymer precipitated outof solution during polymerization. As a result, high-molec-ular-weight PTh8BTD with narrow polydispersity was ob-tained in relatively higher yield. As shown in Figure 1, GPCanalysis (gel permeation chromatography) showed that thenumber-average molecular weight (Mn) of polymerPTh8BTD reached as high as 32 000 (Mw/Mn=1.25). Onthe contrary, polymers PTh6BTD and PTh4TTBTD preci-pitated out during polymerization due to their rigid back-bone and hence lower solubility in refluxing toluene, leadingto low molecular weights and reaction yields. The Mn ofPTh6BTDonly reached 8200 (Mw/Mn=1.39). Themolecularweight of PTh4TTBTD cannot be measured on our GPCbecause of its extremely low solubility in the eluent (THF).

Optical Property. Figure 2 shows the UV-vis absorptionspectra for polymers PTh6BTD, PTh8BTD, andPTh4TTBTD in solution and solid state, respectively. Thedichlorobenzene solutions of all three polymers show broadabsorption bands covering from 350 to 650 nm. The peakmaxima located at ∼550 nm arise from the intramolecularcharge-transfer (ICT) transition.19 These absorption max-ima are accompanied by a strong absorption band at shorterwavelength (∼425 nm) corresponding to theπ-π* transitionof the oligothiophene units.20 The absorption shoulder peakat∼697 nm for PTh4TTBTD solution can be ascribed to theformation of polymer interchain aggregates because thisabsorption peak disappears upon heating.21 Figure 2 alsoshows the absorption spectra of polymer thin films spin-castfromdichlorobenzene solution using a slow solvent evapora-tion process.5 Compared with the solution absorption spec-tra, all three polymers showed pronounced peak broadeningand red shifts of>100 nmof absorption edges upon formingthin films. This phenomenon is attributed to the planariza-tion of aryl rings and the presence of strong interchaininteraction in the solid state.22 It seems that the increase inthe polymer backbone rigidity from PTh8BTD to PTh6BTDand PTh4TTBTD resulted in more pronounced interchaininteraction, leading to more broadening of the absorptionpeaks. It is interesting to point out that the thin films of thethree polymers showed slightly blue-shifted and reducedabsorption upon annealing. This behavior is in sharp con-trast to P3HT, which shows increased and red-shifted ab-sorption upon annealing.6 The polymers synthesized here arequite similar to P3HT, except that an electron-deficientmoiety was incorporated into the main-chain structure.However, their optical properties are quite different fromthat of P3HT, indicating that the introduction of D-Astructure in the polymer backbone has a strong impact onthe interchain interaction.

Thermal Properties. The thermal properties of the poly-mers were investigated by differential scanning calorimetry(DSC). DSC analysis reveals that all three polymers arecrystalline materials and have a strong tendency to crystal-lize, as evidenced by the appearance of the crystallizationpeaks in the DSC cooling scan and the absence of apparentglass transition in the heating scan. This property is bene-ficial for applications in organic solar cells since a highlyordered heterojunction nanostructure enhanced by the crys-tallization of the polymer in a PV active layer is desirable forcharge separation and transport.3,23 As can be seen fromFigure 3, the increase of the benzothiadiazole content in thepolymer backbone from PTh8BTD to PTh6BTD andPTh4TTBTD leads to monotonic increase in both the poly-mer melting point and melting enthalpy. It is worth pointingout that when the bithiophone moieties in polymerPTh6BTD was fused to become thieno[3,2-b]thiophene inpolymer PTh4TTBTD, the melting point increased by 109�C. This indicates that the thieno[3,2-b]thiophene moietysignificantly increased the backbone rigidity and enhancedthe interchain interaction. Perhaps, this is one of the reasonswhy thieno[3,2-b]thiophene semiconducting copolymers ex-hibited superior performance in organic transistors.24

Electrochemical Properties. To investigate the electroche-mical properties of the synthesized polymers and estimatetheir HOMO and LUMO energy levels, cyclic voltammetry(CV) measurement was carried out in a three-electrode cellunder Ar using 0.1MBu4NPF6 in anhydrous CH3CN as thesupporting electrolyte. A Pt disk (1 mm diameter), a Pt wire(0.5 mm diameter), and a silver wire (2 mm diameter) servedas the working electrode, counter electrode, and quasi-reference electrode, respectively. Polymers were coated ontothe working electrode from dichlorobenzene solution. Priorto the CV measurement, the Ag quasi-reference electrodewas first calibrated using a ferrocene/ferrocenium (Fc/Fcþ)redox couple (0.35 V vs Ag/AgCl)25 as an external standardand was found to be-0.02 V vs SCE. Therefore, the LUMOand HOMO energy levels of the polymers can be estimatedusing the following empirical equations: EHOMO=Ep

0 þ4.38 eV and ELUMO=En

0 þ 4.38 eV, respectively, where Ep0

and En0 are the onset potentials for oxidation and reduction

relative to theAg quasi-reference electrode, respectively. Theresults are summarized in Table 1, and the cyclic voltammo-grams are shown in Figure 4. All three polymers underwentreversible cathodic reduction and anodic oxidation, and theCV curves remained almost unchanged after successivemultiple potential scans, indicating high stability of thematerials for both hole and electron injection. For compar-ison purposes, the CV curve of widely used r,r-P3HT wasalso included in Figure 4. As can be seen from Figure 4, allthree polymers show additional redox pairs in the n-dopingprocess owing to the presence of the electron-withdrawingbenzothiadiazole units in the polymer backbone, leading tonarrower energy gaps than P3HT. Meanwhile, the HOMOenergy levels of both PTh6BTDand PTh8BTD remain low at∼5.25 eV, which are important for achieving large open-circuit voltage (Voc) and high air stability. Surprisingly,the HOMO energy level of PTh4TTBTD is relatively high

Table 1. Physical Properties of Oligothiophenes-Benzothiadiazole Copolymers

λmaxabs [nm]a optical gap [eV]b E1/2

ox [V]c E1/2red [V]c HOMO [eV]c LUMO [eV]c energy gap [eV]d Tm [�C]e

PTh6BTD 556 1.78 1.14 -1.10, -1.45 -5.26 -3.37 1.89 192PTh8BTD 537 1.82 1.16 -1.11, -1.49, -1.70 -5.23 -3.30 1.93 143, 154PTh4TTBTD 554 f 1.75 0.93 -1.29, -1.66 -5.03 -3.13 1.90 302

aMeasured in 1,2-dichlorobenzene. bEstimated from the absorption edge of DCB solutions. cDetermined by CV measurements. dEnergy gap =HOMO- LUMO. eDetermined by differential scanning calorimeter from remelt after cooling with a heating rate of 10 �C/min under N2.

fEstimatedfrom hot DCB solution.

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(5.03 eV) probably due to the more coplanar backboneconformation induced by the rigid thieno[3,2-b]thiophenemoieties.

Photovoltaic Properties. The PV response of the synthe-sized low-bandgap polymers PTh6BTD, PTh8BTD, andPTh4TTBTD were investigated using bulk heterojunctionPV cells with a device structure of ITO/PEDOT-PSS/PThnBTD:PC61BM (1:1 w/w)/ LiF (1 nm)/Al (120 nm).The active layerwas spin-cast fromdichlorobenzene solutionusing a slow solvent evaporation process.5 Because of thevery low solubility of PTh4TTBTD in dichlorobenzene atroom temperature, it was difficult to get a dense and uniformfilm, and the resulting device was somewhat short-circuited.Figure 5 shows the current-voltage characteristics of thefabricated PV cells in the dark and under simulated AM 1.5G solar irradiation at 100mW/cm2. PTh8BTD-based deviceshave aVoc of 0.68V, higher thanP3HT-baseddevices (∼0.60V).This can be explained by the higher oxidation potential ofPTh8BTD. PTh6BTD has a HOMO energy level similar toPTh8BTD; however, to our surprise the Voc of PTh6BTD-based devices is only 0.51 V, which is 0.17 V lower than thatof PTh8BTD-based devices. The reason is not very clear atthis moment. Our conjecture is that the relatively poor filmquality of PTh6BTD due to its low solubility caused more

Figure 4. Cyclic voltammograms of PTh6BTD, PTh8BTD,PTh4TTBTD, and P3HT thin films on Pt electrode in 0.1 M Bu4NPF6

acetonitrile solution at 50 mV/min.

Figure 5. Current density-voltage characteristics of (a) PTh6BTD and(b) PTh8BTD solar cells under a simulated AM 1.5 solar irradiation of100 mW/cm2.

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energy loss during charge transport process. This is sup-ported by the fact that the Voc of PTh6BTD-based deviceswas improved to 0.61 V upon annealing. PTh8BTD has avery good solubility in dichlorobenzene even at room tem-perature and can be readily cast into a dense film, leading tohigher shunt resistance and a good fill factor. The deviceperformance is summarized in Table 2. On the contrary toP3HT-based devices, thermal annealing of PThnBTD-baseddevices did not improve the device performance. Indeed, thedevice series resistance increased after annealing, leading tothe degradation of overall device performance. UV-visabsorption study showed that the absorption of the activelayers blue-shifted and decreased in intensity after annealing,as displayed in Figure 6. This result is consistent with thechange in the absorption spectra of polymer neat films uponannealing. We also carried out external quantum efficiency

(EQE) measurement on our devices. As can be seen fromFigure 7, the EQE curves of the devices are similar to theabsorption spectra of the respective active layers except somesmall blue shifts in the polymer PV response peaks. EQEspectra clearly proved that the absorption of low-bandgappolymers PTh6BTD and PTh8BTD in the near-IR region(above 700 nm) contributed to the overall photocurrentgenerated by the devices under illumination. EQE measure-ment also confirmed that the photocurrent decreased afterannealing.

Conclusions

Polythiophene derivatives bearing electron-withdrawing benzo-thiadiazole moieties in themain chainwere synthesized, and theiroptical, thermal, electrochemical, and photovoltaic propertieswere investigated in detail. The broad absorption and good

Table 2. Summary of Device Performance for 1:1 PThnBTD/PCBM (n = 6, 8) Based Solar Cells

donor material shunt resistance (kΩ cm2) series resistance (Ω cm2) Voca (V) Jsc

a (mA/cm2) FFa PCEa (%)

PTh6BTD 10.5b 9.3b 0.51b 5.34b 0.51b 1.39b

15.6c 17.3c 0.61c 3.88c 0.40c 0.95c

PTh8BTD 14.6b 13.5b 0.68b 5.20b 0.49b 1.73b

14.8d 17.2d 0.48d 2.21d 0.30d 0.32d

aUnder simulated AM 1.5 solar illumination at an irradiation intensity of 100 mW/cm2. bFor as-fabricated devices. cFor devices annealed at 110 �Cfor 20 min. dFor devices annealed at 100 �C for 20 min.

Figure 6. Absorption spectra of PThnBTD/PCBM blends before andafter thermal annealing. Figure 7. External quantum efficiency spectra of 1:1 PThnBTD/

PCBM-based PV devices.

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crystalinity demonstrate the potential of these polymers in solarcell applications. Bulk-heterojunction organic solar cells werefabricated with configuration of ITO/PEDOT-PSS/PThnBTD:PCBM (1:1)/LiF/Al and examined under simulated AM 1.5 Gsolar irradiation. Preliminary results showed that PTh8BTD-based solar cells gave better performance with a PCE of 1.73%,aVoc of 0.68 V, a short-circuit current (Jsc) of 5.2 mA/cm2, and afill factor (FF) of 0.49 in the unoptimized devices because of itshigh solar absorption, good solubility, and excellent film-formingproperty. This suggests that we have to make a compromisebetween solubility and high crystallinity when designing newpolymeric materials for organic solar cell applications. In futurework, wewill focus on device optimization to further improve thepower conversion efficiency, such as changing PC61BM loadingratio, optimizing active layer thickness, and replacing PC61BMwith PC71BM for better absorption.

Experimental Section

Materials. All reagents and solvents were purchased fromAldrich and Fisher Chemicals. Anhydrous tetrahydrofuran wasdistilled over sodium/benzophenone under Ar prior to use.

Characterization.NMR spectra were recorded on a 400MHzVarian Unity Inova spectrometer using tetramethylsilane ortrace chloroform in deuterium chloroform as internal refer-ences. Gel permeation chromatography (GPC, Viscotek TDA302) was used for measuring the molecular weight and poly-dispersity index. UV-vis spectra were measured using a VarianCary 50 Spectrometer. Differential scanning calorimetry (DSC)analysis was carried out on a TA Instruments DSC 2920 undernitrogen at a heating/cooling rate of 10 �C/min.

Photovoltaic Device Fabrication and Testing. Glass slidespatterned with ITO (Colorado Concept Coatings LLC) werecleaned by sonicating sequentially in detergent, DI water,acetone, and isopropanol. The active area of each solar celldevice was 5 � 7 mm2. Immediately prior to device fabrication,the ITO substrate was treated in a UV-ozone oven for 15 min.Thereafter, the ITO-coated slides were spin-coated (5000 rpmfor 60 s) with a filtered (0.45 μmNYL with GMF syringe filter)aqueous suspension of poly(ethylene dioxythiophene) dopedwith poly(styrenesulfonic acid), PEDOT:PSS (Baytron P, H.C. Starck), and then baked at 120 �C under N2 for 2 h. Theresulting thin PEDOT:PSS layer was about 50 nm thick. Adichlorobenzene solution of polymer and PCBM (10.0 mg/mLeach) was stirred at 50 �Covernight and then spun-cast on top ofPEDOT-PSS at 600 rpm for 60 s. The film was kept in a Petridish overnight. The thickness of the resulting film was found tobe about 130 and 110 nm for PTh6BTD/PCBM and PTh8BTD/PCBM blend, respectively. The device fabrication was com-pleted by the vacuum deposition of LiF (1 nm) and Al cathode(120 nm). The solar cells with no protective encapsulation weresubsequently tested in air under simulated air mass (AM) 1.5solar irradiation (100mW/cm2, Sciencetech Inc., model SF150).Current-voltage (I-V) characteristics were recorded using acomputer-controlled Keithley 2400 source meter. Externalquantum efficiency (EQE) studies were performed using aXe lamp on a Newport 90015832 monochromator with New-port motorized filter wheel system 74041. The probe beam waschopped at 30 Hz and the signal detected with Newport Merlinradiometry systems 70100 and a Stanford Research SystemsSR570 lock-in amplifier.

Monomer and Polymer Syntheses. The dibromonated mono-mer 4 Th2BTD-2Br was synthesized from commercially avail-able 2,1,3-benzothiadiazole via a divergentmethod according toour recent publication.13 2,5-Bis(trimethylstannyl)thieno[3,2-b]thiophene12 and 5,50-bis(trimethylstannyl)-2,20-bithiophene26

were prepared based on literature procedures.4,7-Bis(3,40,40 0-trioctyl-2,20:50,200-terthiophen-5-yl)-2,1,3-ben-

zothiadiazole (5, Th3BTD). Pd(PPh3)4 (46 mg, 0.04 mmol) was

added to a solution of Th2BTD-2Br (2.14 g, 2.0 mmol) andtrimethyl-(4-hexyl-2-thienyl)stannane (2.15 g, 6.0 mmol) inanhydrous DMF (15 mL). The mixture was stirred under argonfor 24 h at 100 �Cand then allowed to cool to room temperature.SaturatedNaHCO3 aqueous solution and hexanes (50mL)wereadded. The aqueous layer was removed, and the organic layerwas washed three times with distilled water. The organic layerwas dried over magnesium sulfate, and the solvent was removedunder a reduced pressure. The residue was purified by columnchromatography on silica gel (eluent: hexane) to give compound7 (2.16 g, 83%). 1HNMR (400MHz, C6D6, δ): 8.22 (s, 2H), 7.58(s, 2H), 7.32 (s, 2H), 6.64 (s, 2H), 3.00 (t, 4H), 2.85 (t, 4H), 2.45 (t,4H), 1.84 (m, 4H), 1.70 (m, 4H), 1.53-1.26 (m, 64H), 0.95-0.89(m, 18H).

4,7-Bis(50 0-bromo-3,40,400-trioctyl-2,20:50,20 0-terthiophen-5-yl)-2,1,3-benzothiadiazole (6, Th3BTD-2Br). A mixture of Th3-BTD (1.81 g, 1.39 mmol), N-bromosuccinimide (NBS) (0.50 g,2.78 mmol), and silica gel (0.15 g) was dissolved in dichloro-methane (30 mL) and stirred at -10 �C for 5.5 h. The reactionwas quenched with water (100 mL) and then extracted withdichloromethane two times. The combinedCH2Cl2 solutionwaswashed with water and dried over magnesium sulfate. Pureproduct was obtained by a silica gel column using hexane aseluent as a dark solid (1.54 g, 76%). 1HNMR (400MHz, C6D6,δ): 8.20 (s, 2H), 7.58 (s, 2H), 7.26 (s, 2H), 6.92 (s, 2H), 2.98 (t,4H), 2.73 (t, 4H), 2.48 (t, 4H), 1.84 (m, 4H), 1.64 (m, 4H), 1.53-1.45 (m, 8H), 1.40-1.25 (m, 56H), 0.95-0.89 (m, 18H). 13CNMR (100 MHz, C6D6, δ): 153.3, 143.4, 141.5, 141.3, 138.2,136.3, 135.4, 133.0, 131.8, 131.3, 129.6, 127.7, 126.2, 125.7,109.8, 32.6, 31.5, 31.4, 30.5, 30.42, 30.40, 30.3, 30.22, 30.17,30.13, 30.10, 30.04, 30.02, 29.97, 23.46, 14.8, 14.7.

General Procedure for the Preparation of Alternating Copoly-

mers P(ThnBTD). To a solution of appropriate dibromide(0.3 mmol) and 5,50-bis(trimethylstannyl)-2,20-bithiophene (0.3mmol) in anhydrous toluene (6 mL), tetra(triphenylphosphine)-palladium(0) (7 mg, 0.006 mmol) was added in glovebox. Thereaction mixture was stirred at ambient temperature for 5 minand then heated to reflux for 48 h. 2-(Trimethylstannyl)-thiophene (10 mg) was added, and 8 h later 2-bromothiophene(15mg) was added and allowed to react for further 8 h. After thesolutionwas cooled to 75 �C, chlorobenzene (10mL)was added.The polymer was precipitated in methanol and collected byfiltration. A dark purple fiberlike solid obtained was firstwashedwith hexanes on Soxhlet extraction apparatus to removelow-molecular-weight oligomers and then extracted with chlor-obenzene. The chlorobenzene solution was concentrated underreduced pressure and precipitated in methanol. The purifiedpolymer was collected and dried in vacuum oven at 80 �C.

PTh6BTD. 157 mg, 49% yield. GPC: Mn: 8200, Mw/Mn=1.39 (relative to polystyrene standards). Anal. Calcd forC62H78N2S7: C 69.22, H 7.31, N 2.60. Found: C 69.05, H 6.67,N 2.67.

PTh8BTD. 284 mg, yield 63%. GPC: Mn 32 000; PDI 1.25.Anal. Calcd forC86H114N2S9: C 70.54,H 7.84,N 1.91. Found:C70.65, H 7.47, N 2.07.

PTh4TTBTD. 142 mg, 45% yield. Anal. Calcd forC60H76N2S7: C 68.65, H 7.30, N 2.67. Found: C 68.34, H 7.15,N 2.84.

References and Notes

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